Process for the production of polyolefin resin foams

ABSTRACT

This invention provides a process for preparing a polyolefin resin foam, comprising the step of extruding to a low-pressure zone a melt of polyolefin resin containing a volatile blowing agent uniformly dispersed therein, the process being characterized in that the volatile blowing agent is a mixture of 5 to 95% by weight of 1,1,1,3,3-pentafluoropropane and 95 to 5% by weight of a low-boiling aliphatic hydrocarbon having a boiling point of 15° C. or lower. According to the invention, there is obtainable a foam of polyolefin resin which is excellent in heat insulating property, dimensional stability and the like and which can retain them for a long term, using a blowing agent which is free of ozone-depleting risks, unlikely to adversely affect global warming, nonflammable and highly compatible with raw materials for foams.

FIELD OF THE INVENTION

The present invention relates to a process for preparing a polyolefinresin foam and more particularly to a process for preparing an extrudedfoam of polyolefin resin which can retain its excellent heat insulatingproperty for a long term, the process being characterized by use of aspecific volatile blowing agent.

BACKGROUND ART

Widely practiced were processes for preparing a polyolefin resin foamwherein a volatile blowing agent is added to a melt of polyolefin resinsuch as styrene resin and the mixture is extruded to a low-pressurezone.

In such processes, dichlorodifluoromethane (CFC-12),dichlorotetrafluoroethane (CFC-114) and the like have been heretoforemainly used as the volatile blowing agent.

However, in recent years, it was suggested that certain kinds of flons,when released into the atmosphere, would deplete the stratospheric ozonelayer and would cause global warming due to greenhouse effect, therebyinflicting a serious adverse influence on the ecosystem includinghumans. In this situation, an international agreement calls for arestriction of use of flons with a high ozone-depleting risk. SaidCFC-12 and CFC-114 are among the flons to be controlled for restriction.From this viewpoint as well, there is a need for development of novelblowing agents which are free or substantially free from theozone-depleting and global warming problems.

Of late, chlorodifluoromethane and 1-chloro-1,1-difluoroethane wereproposed as flons which would exert little influence on the ozone layer,and they are now coming into use. Yet they contain chlorine in themolecule and are still threatening us with ozone depletion.

Further, the use of chlorine-free fluorohydrocarbon free fromozone-depleting risks was suggested in Japanese Unexamined PatentPublication No.239251/1993 which proposed the use of1,1,1,3,3-pentafluoropropane as a blowing agent for the production ofurethane foams.

Since 1,1,1,3,3-pentafluoropropane (HFC-245fa) is a nonflammablecompound having a boiling point of 15° C. and is a hydrogen-containingfluorohydrocarbon, the compound is considered to entail no or littleozone-depleting or global warming risks. Moreover, HFC-245fa has aboiling point close to that of trichlorofluoromethane (CFC-11)conventionally used as a blowing agent for urethane foams and it isnonflammable. Therefore, HFC-245fa is attracting attention as apromising candidate for blowing agents for urethane which substitute forCFC-11.

However, HFC-245fa exhibits a little high boiling point and a lowsolubility in polyolefin resins, when used as a volatile blowing agentfor the production of foams of polyolefin resins such as styrene resins.Consequently a desirable foam can not be obtained because a gas isreleased in the die during the extrusion foaming under conventionalconditions owing to the low solubility of HFC 245fa.

In this situation, there is a demand for the development of techniquesfor controlling the boiling point and solubility of HFC-245fa in orderto effectively use CFC-245fa as a substitute for CFC-12, CFC-114 or thelike.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 schematically shows an example of apparatus for producing styreneresin foams. In the drawing, indicated at 1 and 2 are extruders, at 3 acooler, at 4 a die and at 5 a passage for feed of a blowing agent.

DISCLOSURE OF THE INVENTION

A principal object of the present invention is to provide a process forpreparing an extruded foam of polyolefin resin which can retain itsexcellent heat insulating property, dimensional stability and the likefor a long term, with the use of a blowing agent which is free ofozone-depleting risks, unlikely to adversely affect global warming,nonflammable, suitable in boiling point, and highly compatible with rawmaterials for foams.

The present inventors conducted extensive research in view of theforegoing prior art problems. The inventors attempted to use low-boilingaliphatic hydrocarbons such as propane, butane or the like in theprocess for extrusion foaming of polyolefin resin in which a volatileblowing agent is used. When propane is used in the process, the obtainedfoam was not imparted a sufficient heat insulating property, whereas theuse of butane resulted in failure to obtain small and stable air bubblesand in pronounced degradation of heat insulating property with time.Namely the attempt failed to produce the contemplated foams. Inventors'further investigations revealed that the above-mentioned object can beachieved by using, as a volatile blowing agent, a mixture of alow-boiling aliphatic hydrocarbon having a boiling point of 15° C. orlower with 1,1,1,3,3-pentafluoropropane (HFC-245fa). The presentinvention was completed based on this novel finding.

According to the present invention, there is provided a process forpreparing a polyolefin resin foam, comprising the step of extruding to alow-pressure zone a melt of polyolefin resin containing a volatileblowing agent uniformly dispersed therein, the process beingcharacterized in that the volatile blowing agent is a mixture of 5 to95% by weight of 1,1,1,3,3-pentafluoropropane and 95 to 5% by weight ofa low-boiling aliphatic hydrocarbon having a boiling point of 15° C. orlower.

It is essential in this invention to use, as a volatile blowing agent inthe extrusion foaming, a mixture of 5 to 95% by weight of1,1,1,3,3-pentafluoropropane (HFC-245fa) and 95 to 5% by weight of alow-boiling aliphatic hydrocarbon having a boiling point of 15° C. orlower.

Specific examples of the low-boiling aliphatic hydrocarbon having aboiling point of 15° C. or lower are methane (b.p.--160° C.), ethane(b.p.--88° C.), ethylene (b.p.--104° C.), propane (b.p.--42° C.),propylene (b.p.--48° C.), butane (b.p.--0.5° C.), butene (b.p.--6° C.),isobutane (b.p.--12° C.), isobutene (b.p.--7° C.), etc. Thesehydrocarbons can be used either alone or in combination.

These low-boiling aliphatic hydrocarbons have a higher solubility instyrene resins or like polyolefin resins than HFC-245fa, and can be usedin a sufficient amount to attain a desired expansion ratio of the foam.Thus, the extruded foams with superior appearance and excellentproperties can be easily prepared. These hydrocarbons used alone show ahigh gas permeability with respect to polyolefin resins and are likelyto diminish the dimensional stability and heat insulating property,whereas they show, when used in mixture with HFC-245fa, a depressed gaspermeability, leading to production of foams which can retain the highdimensional stability and heat insulating property for a long term.Further, in such a mixture, nonflammable HFC-245fa inhibits thecombustion of hydrocarbon which is a flammable component. Consequentlythis feature provides a widely extended composition range of the mixtureas a nonflammable blowing agent.

The mixing ratio of HFC-245fa and low-boiling aliphatic hydrocarbon canbe optionally selected according to the intended application, resinformulation and the like from the ranges of 5-95% by weight of theformer and 95-5% by weight of the latter.

The mixed blowing agent containing HFC-245fa and low-boiling aliphatichydrocarbon can be used either alone or in combination with otherblowing agent. Examples of other blowing agents combinedly usable arelow-boiling halogenated hydrocarbons such as ethyl chloride,difluoromethane, pentafluoroethane, 1,1,1,2-tetrafluoroethane,1,1,1-trifluoroethane and 1,1-difluoroethane, air, inert gases such asnitrogen and carbon dioxide, water, etc.

When other blowing agent is used, the amount of the mixed blowing agentmentioned above is preferably at least 20% by weight, more preferably atleast 40% by weight, based on the total amount of the blowing agents.

The blowing agent for use in this invention may contain a decompositioninhibitor.

Preferred decomposition inhibitors include, for example, nitrobenzene,nitromethane and like nitro compounds, 1,2-butylene oxide,epichlorohydrin and like epoxy compounds, p-t-butyl-catechol,2,6-di-t-butyl-p-cresol and like phenolic compounds, etc.

The amount of the decomposition inhibitor used is about 0.05 to about 5parts by weight per 100 parts by weight of the mixed blowing agentcontaining HFC-245fa and low-boiling aliphatic hydrocarbon.

Either an aromatic polyolefin resin or an aliphatic polyolefin resin canbe used as the polyolefin resin in the process of the invention.

Among them, useful aromatic polyolefin resins include, for example,styrene resins. Usable styrene resins include polystyrene, copolymersprepared by copolymerizing styrene and any of α-methylstyrene, maleicanhydride, acrylic acid, acrylic ester, methacrylic acid, methacrylicester, and the like, polymers prepared by suitably addingstyrene-butadiene rubber (SBR) to polystyrene for modification, etc.Useful aliphatic polyolefin resins are, for example, polyethylene,polypropylene, etc.

The expansion ratio of the foam prepared by the process of the presentinvention is preferably in a suitable range to be described later fromthe viewpoints of retention of heat insulating property, dimensionalstability, and the like. Thus the amount of the blowing agent used issuitably adjusted to achieve the contemplated density of the foam.Usually the total amount of the mixed blowing agent containing HFC-245faand low-boiling aliphatic hydrocarbon and optional other blowing agentis about 3 to about 30 parts by weight per 100 parts by weight of hepolyolefin resin.

According to the invention, it is possible, when required, to addnucleating agents such as talc for control of bubble diameter, coloringagents such as dyes, pigments and the like, lubricants, flameretardants, nonionic surface active agents, etc. in the ranges which donot adversely affect the objects of the invention.

According to the process of the invention, a melt of polyolefin resincontaining the volatile blowing agent uniformly dispersed therein isextruded to a low-pressure zone by the conventional extrusion foamingmethod, whereby a polyolef in resin foam is obtained. For example, rawmaterials comprising a polyolefin resin and optionally the requiredamounts of a nucleating agent, a lubricant, a flame retardant and thelike are fed into an extruder, followed by injection of the specifiedamount of a blowing agent. Then the mixture is melted and kneaded withheating. The melt is extruded, via a slit die or like foaming device, toa low-pressure zone, giving the desired polyolefin resin foam. It isdesirable that the obtained foam have a foam density of about 0.02 toabout 0.045 g/cm³ and an average bubble diameter of about 0.1 to about0.7 mm. The foregoing process gives a plate-like expanded product ofpolyolefin resin having a thickness of e.g. about 10 to about 150 mmwhich is excellent in appearance and properties with a foam density andan average bubble diameter in the above ranges.

When the foam density is less than 0.02 g/cm³, the heat insulatingproperty is appreciably degraded and the expanded product tends toshrink on extrusion, whereas the foam density in excess of 0.045 g/cm³denotes an increased weight and higher costs. Hence it is undesirable.When the average bubble diameter is less than 0.1 mm, the foam densityis difficult to reduce, and the thickness can not be increased, whereasin excess of 0.7 mm, a low density is attained, but a lower heatinsulating property results. Hence it is undesirable.

The process for preparing a polyolefin resin foam according to thepresent invention employs a volatile blowing agent which is free ofozone-depleting risks, unlikely to adversely affect global warming,nonflammable and highly compatible with raw materials for foams, and theobtained expanded product of polyolefin resin is excellent in heatinsulating property, dimensional stability and the like and can retainthe properties for a long term. The foam prepared by the process of theinvention can be suitably used for applications, e.g. as heat insulatingmaterials for building.

BEST MODE FOR CARRYING OUT THE INVENTION

The process for preparing an extruded foam of pololefin resin accordingto the invention is described below in more detail with reference to thefollowing Examples to which alone the invention is not limited. Examples1 and 2 and Comparative Examples 1-3

A foam of styrene resin was prepared by the following process using theapparatus shown in FIG. 1 and comprising an extruder 1, an extruder 2, acooler 3 and a die 4 aligned in this order.

First, 100 parts by weight of styrene resin and 0.5 part by weight oftalc (6 μm in average particle size) were kneaded in the extruder 1 togive a melt. Then the blowing agent in the amount shown in Table 1 wasinjected into the melt from a passage 5 for feed of the blowing agent.The mixture was fed to the die 4 via the extruder 2 and the cooler 3 andforced out to the atmospheric environment from a slit of the die havinga opening width of 1.8 mm to mold a 50 mm-thick plate of polystyrenefoam. The pressure and temperature in the outlets of extruders andcooler were controlled to the conditions shown in Table 1.

The obtained foams were evaluated in respect of appearance, density,average bubble diameter and heat conductivity. The results are shown inTable 1.

                  TABLE 1                                                         ______________________________________                                                         Example                                                                         1          2                                               ______________________________________                                        Pressure/                                                                     Temperature                                                                   Outlet of                                                                     Extruder 1                                                                    Pressure           110        110                                             (kgf/cm)                                                                      Temperature        180        180                                             (° C.)                                                                 Outlet of                                                                     Extruder 2                                                                    Pressure           100        100                                             (kgf/cm)                                                                      Temperature        150        150                                             (° C.)                                                                 Outlet of                                                                     Cooler 3                                                                      Pressure           60         60                                              (kgf/cm)                                                                      Temperature        110        110                                             (° C.)                                                                 Amount of                                                                     Blowing                                                                       Agent                                                                         (wt. part)                                                                    HFC 245fa          4.8        12.1                                            Propane            6.3                                                        Isobutane                     5.2                                             Expanded product                                                              Appearance         Excellent  Excellent                                       Density            0.035      0.035                                           (g/cm.sup.3)                                                                  Av.Bubble Diameter(mm)                                                                           0.4        0.4                                             Heat Conductivity                                                             (kcal/mHr° C.)                                                         after 7 days       0.0220     0.0215                                          after 100 days     0.0235     0.0220                                          Remarks                                                                       ______________________________________                                                     Comparative Example                                                             1          2         3                                         ______________________________________                                        Pressure/                                                                     Temperature                                                                   Outlet of                                                                     Extruder 1                                                                    Pressure       110        110       110                                       (kgf/cm)                                                                      Temperature    180        180       180                                       (° C.)                                                                 Outlet of                                                                     Extruder 2                                                                    Pressure       100        100       100                                       (kgf/cm)                                                                      Temperature    150        150       150                                       (° C.)                                                                 Outlet of                                                                     Cooler 3                                                                      Pressure                                                                      (kgf/cm)       60         60        60                                        Temperature                                                                   (° C.)  110        110       110                                       Amount of                                                                     Blowing                                                                       Agent                                                                         (wt. part)                                                                    HFC 245fa      24                                                             Propane                   7.9                                                 Isobutane                           10.4                                      Expanded product                                                              Appearance     Poor       Fair      Fair                                      Density        --         0.025     0.030                                     (g/cm.sup.3)                                                                  Av.Bubble Diameter(mm)                                                                       --         0.7       0.5                                       Heat Conductivity                                                             (kcal/mHr° C.)                                                         after 7 days              0.0260    0.0225                                    after 100 days            0.0285    0.0250                                    Remarks        Nonfoamable                                                                   for                                                                           separation                                                     ______________________________________                                    

As apparent from the results, the process of the present invention givesan extruded foam of styrene resin having a good surface appearance andcapable of retaining the excellent heat insulating property for aprolonged period of time.

What is claimed is:
 1. A process for preparing a polyolefin resin foam,comprising the step of extruding to a low-pressure zone a melt ofpolyolefin resin containing a volatile blowing agent uniformly dispersedtherein, the process being characterized in that the volatile blowingagent is a mixture of 5 to 95% by weight of 1,1,1,3,3-pentafluoropropaneand 95 to 5% by weight of a low-boiling aliphatic hydrocarbon having aboiling point of 15° C. or lower.
 2. The process according to claim 1,wherein the low-boiling aliphatic hydrocarbon is at least one memberselected from the group consisting of methane, ethane, ethylene,propane, propylene, butane, butene, isobutane, and isobutene.
 3. Theprocess according to claim 1, wherein the polyolefin resin ispolystyrene, polyethylene or polypropylene.